1. Field of the Invention
This invention relates to a ceramic heater having a heat resisting element embedded in a ceramic body of nonoxide.
2. Prior Art
In a conventional type ceramic heater, as shown in FIG. 1, a resistance heating pattern 3 of an optional shape such as a sinuous shape, spiral shape and of width and length is formed by a so-called thick film forming method such as screen printing by use of paste prepared by kneading manganesemolybdenum, molybdenum, tungsten or the like powder so that the opposing face of either of a pair of upper and lower substrates 1 and 2 of a ceramic green sheet made of alumina as a raw material may have a specified electric resistance value. The pair of upper and lower substrates 1 and 2 are laminated so as to form welded portions for lead wire terminals into a desired shape such as a flat plate or a cylindrical shape shown in FIG. 2 either by laying the upper and lower substrates 1 and 2 one over the other with the resistance heating pattern 3 sandwiched therebetween and cutting away the substrate 2 in part so that both terminal ends 3' (the other end not shown) of the resistance heating pattern 3 may be exposed, or by laying either one of the substrates 1 and 2 over the other in an offset relation, or by forming through holes in the substrate 2 after the substrates 1 and 2 have been laminated. Thereafter, the substrates 1 and 2 thus formed into the desired shape are sintered into one body in a reduced atmosphere of about 1600.degree. C. The initial end 3' having the resistance heating pattern 3 exposed thereat and the terminal end portion not shown are plated with nickel and have lead terminals 4 fixed thereto by silver soldering. By energizing the terminals 4 with an electric current the heat resisting element embedded in the alumina ceramic is heated. The ceramic heater constructed in the manner described above finds application in all fields of industry.
But the ceramic heater having such a heating element embedded in the alumina ceramic body is not free from the disadvantage of being low in thermal shock resistance, and for example, heaters each having the resistance heating element embedded in a plate-shaped alumina ceramic body of a dimension of 30 mm long.times.10 mm wide.times.3 mm thick were energized and kept heated to various degrees of temperature and the heaters thus heated were immersed in the water of 25.degree. C. to examine the temperature at which cracks were produced, only to find that all the ceramic heaters produced cracks in the yemperatures ranging from 200.degree. to 240.degree. C. such that they were impossible to use.
Also, a heater having a resistance heating element embedded in an alumina ceramic body formed into a cylinder 50 mm in diameter, the resistance heating element having tungsten paste printed thereon, was tested to see a rise time necessary for room temperature (20.degree. C.) to be elevated to 800.degree. C. (temperature in the highest temperature portions). The result of the test showed that cracks were produced in a rise time less than five seconds and that the heater made of alumina ceramic was weak in thermal shock resistance.
Furthermore, in alumina ceramic, mechanical strength at high temperatures, namely high temperature deflection strength is as small as 20-30 kg/mm.sup.2 in the range of room temperature to 900.degree. C., which is insufficient in strength at high temperatures. Also, the heater in which alumina ceramic was used was found unable to provide a ceramic heater having a stable high temperature heating characteristic as a result of the test conducted on the change in resistance value which the resistance heating element has undergone under the effect of time. The resistance heating element formed by the thick film forming method as above and embedded in the alumina ceramic was subjected to a repeated test in the manner that, after the element was maintained at a saturation temperature of higher than 1000.degree. C. for about 30 seconds, power was off and after a lapse of 60 seconds the element was again heated to the saturation temperature. Examination of changes in the resistance value of the resistance heating element due to the effect of time by the repetition of tests showed that when the repetition of the above procedure 1500 times at a saturation temperature of 1000.degree. C. was effected, the element increased about 10% in resistance value and that when the repetition was carried out 1500 times at a saturation temperature of 1100.degree. C., the element increased about 20 to 30% in resistance value. In the manner described, because the resistance heating element changed in resistance value during its use as a heater, application of the same voltage effected a gradual decrease in heating value and could not provide a specified heating temperature.